Extreme ultraviolet (“EUV”) light, e.g., electromagnetic radiation having wavelengths of around 50 nm or less, e.g., 5-50 nm, and including light at a wavelength of about 13.5 nm, can be used in lithography processes to produce extremely small features in substrates, e.g., silicon wafers.
Methods to produce EUV light include, but are not necessarily limited to, converting a material into a plasma state that has an element, e.g., xenon, lithium or tin, with an emission line in the EUV range. The plasma is typically formed in a vacuum chamber and light produced in the chamber may then be collected and directed to a focus for subsequent utilization outside of the light source chamber, e.g., for semiconductor integrated circuit manufacturing lithography.
In one such method, often termed electric discharge produced plasma (“DPP”), the plasma may be produced by an electrical discharge between a pair of electrodes. In another method, the required plasma can be produced by irradiating a target material, such as a droplet, stream or cluster of material having the required line-emitting element, with a laser beam. This later process is referred to as laser produced plasma (“LPP”).
For each of these processes, the EUV light produced is typically monitored using various types of metrology equipment. This monitoring is often performed at, or downstream of a collection point, e.g., focal point, and may be conducted while the EUV light utilizing tool, e.g., lithography tool, is on-line (i.e., using light from the light source), for example, by sampling a portion of the light stream. In addition, it may be desirable to analyze most or all of the EUV light produced, for example, while the EUV light utilizing tool is off-line (i.e., not using light from the light source). Unfortunately, the lack of a practical beam splitter for EUV radiation has hindered metrology efforts thus far. Another factor that may complicate metrology efforts is the large amount of debris that may be generated by the plasma in the vacuum chamber. These undesirable by-products can include, electrons, high energy ions and scattered debris from the plasma formation, e.g., neutral atoms/molecules and/or clumps of source material that is not fully ionized in the plasma formation process.
These plasma formation by-products can potentially damage or reduce the operational efficiency of the various metrology components including, but not limited to, mirrors used to direct EUV light to a detector including multi-layer mirrors (MLM's) capable of EUV reflection at normal incidence and grazing angle incident mirrors, and the exposed surfaces of metrology detectors including filtering foils. The high energy ions and/or source material debris may be damaging to the optical elements in a number of ways, including heating them, coating them with materials which reduce light transmission, penetrating into them and, e.g., damaging structural integrity and/or optical properties, e.g., the ability of a mirror to reflect light at such short wavelengths, corroding or eroding them, diffusing into them and/or sputtering surface material.
In addition to debris, another factor that may need to be considered when measuring or directing relatively high intensity, e.g., focused, EUV light is the heat generated when the EUV light contacts a component surface. In this regard, certain multi-layer mirrors (MLM's), which may function to reflect and/or filter EUV radiation, may experience performance degradation if exposed to excessive heat.
It is to be appreciated that accessing optical elements including metrology equipment in the plasma chamber that have been contaminated or damaged by heat for the purpose of cleaning or replacing the elements can be expensive, labor intensive and time-consuming. In particular, these systems typically require a rather complicated and time consuming purging and vacuum pump-down of the plasma chamber prior to a re-start after the plasma chamber has been opened. This lengthy process can adversely affect production schedules and decrease the overall efficiency of light sources for which it is typically desirable to operate with little or no downtime.
With the above considerations in mind, Applicants disclose systems and methods for EUV light source metrology.